1. Field of the Invention
The present invention relates to a sensor circuit for detecting a change in pressure or amount of light, which is detected by a pressure sensor, light sensor etc., as a change in resistance value, and outputting the change as a voltage change.
2. Description of the Related Art
Various sensors exist, such as pressure sensors and light sensors. A pressure sensor detects, for example, the weight of a turntable in a microwave oven and is used to convert this weight into an electrical signal. A light sensor is used to convert information that has been captured by an imaging device into an electrical signal.
Such a sensor circuit is known to comprise sensor elements for receiving pressure or light and representing the received pressure or light as a change in resistance value, and an amplifier circuit for producing a voltage value by amplifying the change in resistance value of the sensor elements as a voltage value. An output signal of the amplifier circuit, having the voltage value obtained from the amplifier circuit, is transmitted to an analog-to-digital converter. The analog-to-digital converter outputs the voltage value of the output signal from the amplifier circuit as digital data. In this manner, a pressure or amount of light is transmitted to peripheral circuits as digital data enabling various processing to take place.
Sensor circuits have various uses, such as those described above, and there are also various operating conditions for a device containing a sensor circuit. Particularly, the precision of a sensor circuit is influenced by the temperature of the environment in which it is used.
For example, there is a so-called drift characteristic where the resistance value of the sensor element varies depending on temperature. Generally, the range of variation in resistance value of the sensor elements widens with an increase in drift characteristic. In order to maintain the conversion range of an analog-to-digital converter with such a drift characteristic, it is necessary to reduce the amount of current flowing in the sensor elements or to limit the range of voltage output from the sensor by lowering sensitivity. As a result the drift characteristic hinders effective use of the resolution possessed by the analog-digital converter.
The effect of the drift characteristic will be described with reference to FIG. 9, which is a drawing showing the variation in the resistance value of a light sensor element with an increase in the amount of light, in the case of a sensor circuit for detecting variations in amount of light. In FIG. 9, the vertical axis represents sensor element resistance value and the horizontal axis represents light amount, with straight line A showing variations for a lower limit temperature and straight line A' showing variations for an upper temperature limit. Also, R0 is the resistance value when the light amount received by the sensor element at the lower temperature limit =0 (initial resistance value at lower temperature limit), and Rm is the resistance value when the light amount received by the sensor element at the lower temperature limit is a maximum. Similarly, R0' is the resistance value when the light amount received by the sensor element at the upper temperature limit =0 (initial resistance value at upper temperature limit), and Rm' is the resistance value when the light amount received by the sensor element at the upper temperature limit is a maximum. The range shown by X is preferable as a conversion range actually required by an analog-to-digital converter. However, since, due to the drift characteristic, a range can be from that of the straight line A to that of the straight line A' depending on the temperature, it is necessary for the conversion range of the analog converter to be the range shown by X'. This means that the conversion range of the analog-to-digital converter is increased and the sensitivity of the analog-to-digital converter per single bit is deteriorated.